Fluoroscopic x-ray images play a key-role in a variety of surgical procedures, e.g., fracture reduction, pedicle screw insertion and implant positioning for treating hip fractures. The surgeon uses the mobile fluoroscopic x-ray machine (C-arm) in the operation room (OR), to determine the position and orientation of bones, implants and surgical instruments. X-ray fluoroscopy instruments have several limitations, one of which is a narrow field of view (FOV) which prevents imaging a large region of interest (ROI), e.g., in the case of long implants placement. A way to address this issue is to acquire several individual overlapping images of the ROI and to compose the equivalent of a single x-ray image by finding adequate correlation between the single images. The final panoramic (or mosaic) image may therefore be several times wider than the original field of view of the apparatus.
Panoramic x-ray views can be useful during various stages of many orthopedic surgery procedures. Preoperatively, they serve for diagnosis and measurements. Intraoperatively, where they are particularly useful, they help avoid positioning errors and enable the surgeon to have a global, unornamented vision of the ROI. Postoperatively, they can also provide useful information concerning the outcome of the surgery.
Panoramic X ray imaging is useful in other areas of medical imaging as well, including cardiovascular angiography of long blood vessels and Digital Radiography (DR) of skeleton parts such as the spine or legs. Modem DR is based on digital X ray detectors which are typically limited in coverage to 43 cm. Longer anatomies are imaged by taking several overlapping images and stitching them together.
Creating panoramic views out of individual images is known in the prior art, and constitutes a very active field of research in the domain of graphic computing. Techniques for composing a panoramic image are disclosed for example in U.S. Pat. No. 5,262,856 and a method for automatic alignment of individual overlapping pictures can be found in U.S. Pat. No. 5,649,032. In brief, generation of panoramic images requires three distinct steps: 1) correcting the distortion of each single image, generally caused by the optical system, 2) alignment and stitching of the individual images and 3) composing the final panoramic image. Several publications relate to methods for correcting optical distortion in x-ray digital imaging such as U.S. Pat. Nos. 4,736,399 or 6,618,494. However, the most difficult step remains aligning and stitching the individual pictures.
To illustrate geometrical difficulties in generating a panoramic image we consider an x-ray imaging system 100 as shown schematically in FIG. 1, x-ray source 102 emits conical beam 104 which is received by area detector 106. System 100 is used to generate one X ray image at a first position and then both the source 102 and detector 106 are translated laterally by a distance X and used to generate a second image at a second position. Objects A,B,C and D represent features of interest in the imaging field. Object A in plane 108 is in the overlap region of the two images so it appears in both images. In order to stitch the images, object A may be identified in each of the two images and the images translated relative to each other till the object accurately overlaps in the two images. Under these conditions other features in plane 108 will appear focused in the stitched panoramic image. However, object B in plane 110 will appear blurred or doubled in the stitched image, and object C and D in plane 110 (which are not in overlap region) will appear focused but at the wrong distance between them.
In general, image alignment can only be achieved at a certain distance from the x-ray source, a surface known as the plane of interest (POI). Despite its name, The POI is not limited to a single spatial plane, for example, in a whole leg panoramic image, it is sometimes useful to stitch along both the (possibly different) femur and the tibia planes. In fact, the POI can be any continuous spatial surface between the x-ray source and the detector. Stitching of images according to a certain POI causes objects out of the POI to be “blurred”, a phenomenon commonly called parallax error.
The physical overlap between the images, the geometric constraints on imaging system position, and the type of mapping between images are considered to be the major parameters influencing the stitching procedure (Ziv et Josckowicz, IEEE Trans. on Med. Im., 23(1):1-9). Information related to imaging system translation is either provided by the motion control system in automated motion, or by image analysis methods. Early prior art stitching methods which were based on image analysis rely on identification of specific features in consecutive pictures. However, detection and alignment of prominent anatomical features in x-ray fluoroscopic imaging is considered unreliable and occasionally inaccurate. Therefore, other methods using artificial markers specifically positioned within the FOV were developed, as for example described in EP 1 632 181. When one or several markers, being localized precisely in the plane, are present in two consecutive images, the exact translation can be computed accordingly.
Alternatively, Ziv and Joskowicz propose a method for focusing the panoramic image about the true ROI using a manual phase in which the user indicates the edges that are to be “in focus” and the POI is set accordingly [Ziv et Josckowicz, IEEE Trans. on Med. Im., 23(1):1-9].
A software and hardware package commercially called “SmartStitch” is distributed by CMT Medical Technology Ltd. of Yogne'ham, Israel. The package allows for acquisition of several digital x-ray images wherein the x-ray source and detector are moved in parallel relative to the patient between shots and the patient is held still. An x-ray ruler operative to generate ruler marks on the X ray images is placed alongside the patient. In order for the stitching to be successful, the ruler must be positioned at the same distance from the source as the anatomic features of interest, for example spine or legs. The stitching operation involves alignment of the ruler marks in the regions of overlap between successive images. The resulted panoramic image provides a focused accurate image for features in the plane of the ruler but not in other planes. SmartStitch does not provide a solution for cases where the POI and anatomic features therein are tilted relative to the imaging system and it's motion. Similar packages are provided by other vendors of DR and CR systems.
Some other solutions to obtain panoramic views by stitching multiple X-ray images can be found in the following patents:                EP 0 655 861 and corresponding U.S. Pat. No. 6,097,833 provides an image composition method by overlapping a series of consecutive sub-images acquired by an x-ray source and image intensifier translated along the length of the patient. In order to match accurately consecutive sub-images in the stitching process, the shift between two positions of the imager is determined by finding maximum correlation between the pixel-values in overlapping portions. The above references also suggest the use of an x-ray ruler embedded in the table on which the patient lies. However, they do not propose a solution of how to reconstruct focused images at any POI.        EP 1 255 403 relates to an image composition method for use in digital x-ray imaging. The apparatus consists of a mobile detector, that can be translated along an axis in front of a static x-ray source having a specific collimator enabling the synchronized orientation of the cone beam with the detector positions. The same effect can be obtained by variably tilting the X ray source as the detector is moved without changing the focal spot position. This method eliminates the stereovision-like geometric distortion but cannot be implemented in standard C-arm equipment since on a C arm the source and the detector move together.        U.S. Pat. No. 5,123,056 to Wilson provides a processing and display technique for panoramic images of a whole leg in peripheral angiography setting, based on processing and display of high resolution images and low resolution images, wherein alignment of overlapping images is achieved by a manual or automatic optimization of the visual appearance of features in the POI.        U.S. Pat. No. 5,833,607 to Chou et. al. and U.S. Pat. No. 6,101,238 to Murthy et. al., both assigned to Siemens AG, Germany described other image composition methods for use in peripheral angiography. The x-ray examination apparatus consists of a motorized C-arm which acquires overlapping images by simultaneous translation of the x-ray source and the image intensifier. The images are processed to emphasize certain features and are aligned by detecting and matching meaningful features on a reconstruction plane.        
This survey of the prior art shows that most of the methods used for creating panoramic views based on multiple x-ray images are based on expensive and bulky systems that cannot be used intraoperatively. Moreover, some methods developed specifically for x-ray fluoroscopic panoramic imaging, preferably use the position of the x-ray source of the apparatus as reference for stitching consecutive images. All of these systems at least partially correct the parallax error at a specific plane, but the POI is hard to manipulate. Also, these systems are unable to provide a metrical system that can enable the surgeon to measure features lengths and orientations on the resulting image
It is therefore an object of this invention to provide a system that can be used to obtain x-ray panoramic views of a region of interest, larger than the field of view of the imaging apparatus.
It is another object of this invention to provide a system that can provide such x-ray panoramic views of a region of interest, focused at a POI adjustable by the operator according to the anatomy of interest.
It is still another object of this invention to provide a method for stitching multiple x-ray images and compose a panoramic view, based on specific features appearing in consecutive images where the specific features are not necessarily positioned in the POI.
It is still another object of this invention to provide a system that can be adapted to a mobile C-arm fluoroscopic machine in an operating room to obtain panoramic views intraoperatively.
It is still another object of this invention to provide the possibility of performing metric measurements in the reconstruction plane.
It is a further object of this invention to provide the possibility of inserting an additional image in a composed panoramic image.
Other objects and advantages of present invention will appear as the description proceeds.